At which point Fateweaver collapses on his T5 4+ butt and the plan goes tits-up.
Well, yeah, but running a Cursed Earth near Fatey gives him a very respectable 3++ rerolling 1s. The point of that combo is that it makes your stuff hard to kill.
Yeah, you can get to 3++, but you could do that throughout 6th with a Skyshield and Mark of Tzeentch and nobody did it so I highly doubt it'll break the game.
And you still can on units like Havocs. My FLGS wasn't happy with that interpretation of the rules, saying MoT didn't apply to an external save like the Skyshield's (which I think is rubbish, but I'm not going to alienate my local gaming group and don't want to be That Guy, so I'll go along with it). The nice thing about these 3++es is that they're mobile, and so can go on assaulty units.
So... a hugely expensive unit of dudes who are as tough as 14pt Marines who rely on a random Psychic Power with a 12" range to be useful, the purpose of which is to cause a Blind check (which most armies will pass or not care about), and then charge people despite having no grenades so they strike last? I think that's a really awful plan, but if you can find a way to make it work then by all means enlighten me (I know I sound cynical, but I honestly do want Warp Talons to be even vaguely viable - them models and fluff are great!)
They are in no way competitive. They are, however, much more reliable and can take advantage of certain special rule opportunities they did not have before. They've always been something to run in non-competitive lists, and so this nice little synergy, so perfectly paired to their weaknesses, makes them more effective, less fragile, and therefore possibly even viable.